Spent an inordinate amount of time solving a problem last week. A site with multiple forums, forums were hosted inside an iframe (but under the same domain).

Problem: trying to go into administration panel of the board would either crash the browser or cause an internal server error. Hmmph.

Solution: Auto cookies, nice likkle script which suggested the right settings for me. http://www.phpbb.com/community/viewtopic.php?t=228741